Teresa R. Franklin

Limbic Activation to Cigarette Smoking Cues Independent of Nicotine Withdrawal: A Perfusion fMRI Study

Exposure to cigarette smoking cues can trigger physiological arousal and desire to smoke. The brain substrates of smoking cue-induced craving (CIC) are beginning to be elucidated; however, it has been difficult to study this state independent of the potential contributions of pharmacological withdrawal from nicotine. Pharmacological withdrawal itself may have substantial effects on brain activation to cues, either by obscuring or enhancing it, and as CIC is not reduced by nicotine replacement strategies, its neuro-anatomical substrates may differ. Thus, characterizing CIC is critical for developing effective interventions. This study used arterial spin-labeled (ASL) perfusion fMRI, and newly developed and highly appetitive, explicit smoking stimuli, to examine neural activity to cigarette CIC in an original experimental design that strongly minimizes contributions from pharmacological withdrawal. Twenty-one smokers (12 females) completed smoking and nonsmoking cue fMRI sessions. Craving self-reports were collected before and after each session. SPM2 software was employed to analyze data. Blood flow (perfusion) in a priori-selected regions was greater during exposure to smoking stimuli compared to nonsmoking stimuli (p<0.01; corrected) in ventral striatum, amygdala, orbitofrontal cortex, hippocampus, medial thalamus, and left insula. Perfusion positively correlated with intensity of cigarette CIC in both the dorsolateral prefrontal cortex (r2=0.54) and posterior cingulate (r2=0.53). This pattern of activation that includes the ventral striatum, a critical reward substrate, and the interconnected amygdala, cingulate and OFC, is consistent with decades of animal research on the neural correlates of conditioned drug reward.

Prelude to passion: limbic activation by "unseen" drug and sexual cues.

Background The human brain responds to recognizable signals for sex and for rewarding drugs of abuse by activation of limbic reward circuitry. Does the brain respond in similar way to such reward signals even when they are “unseen”, i.e., presented in a way that prevents their conscious recognition? Can the brain response to “unseen” reward cues predict the future affective response to recognizable versions of such cues, revealing a link between affective/motivational processes inside and outside awareness? Methodology/Principal Findings We exploited the fast temporal resolution of event-related functional magnetic resonance imaging (fMRI) to test the brain response to “unseen” (backward-masked) cocaine, sexual, aversive and neutral cues of 33 milliseconds duration in male cocaine patients (n = 22). Two days after scanning, the affective valence for visible versions of each cue type was determined using an affective bias (priming) task. We demonstrate, for the first time, limbic brain activation by “unseen” drug and sexual cues of only 33 msec duration. Importantly, increased activity in an large interconnected ventral pallidum/amygdala cluster to the “unseen” cocaine cues strongly predicted future positive affect to visible versions of the same cues in subsequent off-magnet testing, pointing both to the functional significance of the rapid brain response, and to shared brain substrates for appetitive motivation within and outside awareness. Conclusions/Significance These findings represent the first evidence that brain reward circuitry responds to drug and sexual cues presented outside awareness. The results underscore the sensitivity of the brain to “unseen” reward signals and may represent the brains primordial signature for desire. The limbic brain response to reward cues outside awareness may represent a potential vulnerability in disorders (e.g., the addictions) for whom poorly-controlled appetitive motivation is a central feature.

DAT Genotype Modulates Brain and Behavioral Responses Elicited by Cigarette Cues

We previously demonstrated differential activation of the mesocorticolimbic reward circuitry in response to cigarette cues independent of withdrawal. Despite robust effects, we noted considerable individual variability in brain and subjective responses. As dopamine (DA) is critical for reward and its predictive signals, genetically driven variation in DA transmission may account for the observed differences. Evidence suggests that a variable number of tandem repeats (VNTRs) polymorphism in the DA transporter (DAT) SLC6A3 gene may influence DA transport. Brain and behavioral responses may be enhanced in probands carrying the 9-repeat allele. To test this hypothesis, perfusion fMR images were acquired during cue exposure in 19 smokers genotyped for the 40 bp VNTR polymorphism in the SLC6A3 gene. Contrasts between groups revealed that 9-repeat (9-repeats) had a greater response to smoking (vs nonsmoking) cues than smokers homozygous for the 10-repeat allele (10/10-repeats) bilaterally in the interconnected ventral striatal/pallidal/orbitofrontal cortex regions (VS/VP/OFC). Activity was increased in 9-repeats and decreased in 10/10-repeats in the VS/VP/OFC (p<0.001 for all analyses). Brain activity and craving was strongly correlated in 10/10-repeats in these regions and others (anterior cingulate, parahippocampal gyrus, and insula; r2=0.79–0.86, p<0.001 in all regions). Alternatively, there were no significant correlations between brain and behavior in 9-repeats. There were no differences in cigarette dependence, demographics, or resting baseline neural activity between groups. These results provide evidence that genetic variation in the DAT gene contributes to the neural and behavioral responses elicited by smoking cues.

The GABA B agonist baclofen reduces cigarette consumption in a preliminary double-blind placebo-controlled smoking reduction study.

The surge in dopamine in ventral striatal regions in response to drugs of abuse and drug-associated stimuli is a final common pathway of addiction processes. GABA B agonists exert their effects indirectly, by quieting dopaminergic afferents. The ability of the GABA B agonist, baclofen to ameliorate nicotine and drug motivated behavior is established within the animal literature, however its potential to do so in humans is understudied, particularly with respect to its possible utility as a smoking cessation agent. We conducted a nine-week double-blind placebo-controlled pilot trial of baclofen for smoking reduction (N=30/group) in smokers contemplating, but not quite ready to quit. Baclofen was titrated upwards to 20mg q.i.d. over a period of twelve days. The primary outcome measure was the number of cigarettes smoked per day (CPD). A significant group by time effect of medication was observed. Baclofen was superior to placebo in reducing CPD (beta=0.01, t=1.97, p<0.05). The most common side effect reported during baclofen treatment is transient drowsiness, however there were no differences between groups in mild, moderate, or severe sedation. Craving was significantly lowered at end of treatment in all smokers (p<0.02). Retention did not differ between groups. In line with a multitude of preclinical studies examining the effects of baclofen on drug-motivated behavior, baclofen reduced CPD. In agreement with other studies examining craving and drug use, reductions in CPD were accompanied by a reduction in craving, a major motivator underlying continued smoking and relapse. These preliminary results demonstrate provisional evidence of the utility of baclofen to aid in smoking cessation and indicate further investigation.

Retrospective study: Influence of menstrual cycle on cue-induced cigarette craving

Cigarettes acquire reinforcing properties from nicotine and from cues associated with their intake. However, smoking in males and females may be reinforced differentially. Smoking in females is posited to be influenced more by cues whereas male smoking is influenced predominantly by the direct pharmacological actions of nicotine in the brain. Menstrual cycle phase may contribute to some of the sex differences observed in smokers. We hypothesized that females may report more intense craving to smoking cue exposure than males and, further, that female craving scores may be influenced by menstrual cycle phase. Thus, we reexamined previously collected cue exposure data with respect to sex and cycle phase. Self-report measures were collected from subjects prior to and immediately following exposure to visual smoking stimuli. The study included 69 male and 41 female treatment-seeking subjects who smoked more than 15 cigarettes per day for more than 10 years. Females were grouped according to cycle phase. Of the female subjects, 17 were classified as follicular phase females (FFemales) and 24 were classified as luteal phase females (LFemales). Change scores were calculated from the subjective data collected before and after stimulus presentation. Contrary to our hypothesis, overall, males and all females did not differ in their level of cue-induced craving; however, when females were separated into groups by cycle phase, FFemales reported significantly less craving than either males or LFemales (p<.05). The suppressed craving response in FFemales suggests an influence of cycle phase on cue-induced craving.

Menstrual Cycle Phase at Quit Date Predicts Smoking Status in an NRT Treatment Trial: A Retrospective Analysis

BACKGROUND AND OBJECTIVE The deleterious health consequences of smoking are even more severe for women, yet ironically, they have more difficulty quitting than men. Identifying relapse predictors for women and implementing strategies to increase their chances of successfully quitting and remaining abstinent are important goals. Clinicians and researchers suggest that women could achieve greater success in smoking cessation interventions if the initial quit attempt coincided with the follicular phase (i.e., preovulatory phase) of their menstrual cycle (MC) rather than the luteal phase (i.e., premenstrual). However, no experimental data have been published to support this claim. Our objective was to determine whether MC phase affected smoking status in premenopausal female smokers participating in a smoking cessation treatment trial. METHODS Data from 102 treatment-seeking smokers who participated in an 8-week nicotine replacement therapy (NRT) plus behavioral intervention smoking cessation study were examined retrospectively. NRT began the day subjects attempted to quit smoking (quit date). For analyses, smokers were grouped according to sex, and women were subdivided by MC phase at quit date into follicular (FF, days 1-14, n = 16) and luteal (LF, days 15-30, n = 21) groups. RESULTS Smoking status was examined on the third day after the quit date (day 3) and at 1 week posttreatment (week 9). On day 3, 52% of LFs reported smoking compared with 19% of FFs (p < 0.04), and at week 9, 71% of LFs reported smoking compared with 31% of FFs (p < 0.02). In a comparison group of men (n = 65), 25% were smoking at day 3 and 68% at week 9. Self-report at week 9 was verified by urine cotinine levels. CONCLUSIONS These data support the supposition that better treatment outcomes can be achieved by scheduling quit dates to coincide with the follicular phase of the MC in female smokers.

Dopamine transporter genotype modulation of neural responses to smoking cues: confirmation in a new cohort

Previously we demonstrated profound effects of dopamine transporter (DAT) SLC6A3 genotype on limbic responses to smoking cues (SCs). Probands carrying at least one copy of the 9‐repeat allele (9‐repeat carriers) had greater neural responses to SCs in the anatomically interconnected rostral ventral striatum/medial orbitofrontal cortex (VS/mOFC), compared with homozygotes for the 10‐repeat allele (10/10‐repeats). To test the reliability of the initial findings, we examined perfusion functional magnetic resonance images acquired during SC exposure in a new cohort of smokers (N = 26) who were genotyped for the SLC6A3 polymorphism. In smokers overall, activity was enhanced in the VS/mOFC (t = 3.77). Contrasts between allelic groups revealed that 9‐repeat carriers had a greater response to SCs in the VS (t = 3.12) and mOFC (t = 3.19). In separate groups, 9‐repeat carriers showed increased activity in the VS (t = 5.47) and mOFC (T = 4.96), while no increases were observed in 10‐repeats. Subjective reports of craving correlated with increased activity in reward‐related structures including the extended amygdala, insula and post‐central gyrus, and decreased activity in the dorsolateral prefrontal cortex, and were DAT‐genotype dependent (r = 0.63–0.96). In secondary analyses, we found that The Fagerström Test for Nicotine Dependence scores correlated with enhanced SC‐induced perfusion in 10/10‐repeats in the insula, mOFC, medial temporal and superior frontal gyri (r = 0.50–0.82), while correlations were absent in 9‐repeat carriers. Despite heterogeneity introduced by a host of factors, including variance in other genes involved in smoking behavior, we confirm that DAT genotype predicts the direction and location of neural responses to SCs.

The Effects of Chronic Cigarette Smoking on Gray Matter Volume: Influence of Sex

Cigarette smoke contains nicotine and toxic chemicals and may cause significant neurochemical and anatomical brain changes. Voxel-based morphometry studies have examined the effects of smoking on the brain by comparing gray matter volume (GMV) in nicotine dependent individuals (NDs) to nonsmoking individuals with inconsistent results. Although sex differences in neural and behavioral features of nicotine dependence are reported, sex differences in regional GMV remain unknown. The current study examined sex differences in GMV in a large sample of 80 NDs (41 males) and 80 healthy controls (41 males) using voxel-based morphometry. Within NDs, we explored whether GMV was correlated with measures of cigarette use and nicotine dependence. High-resolution T1 structural scans were obtained from all participants. Segmentation and registration were performed in SPM8 using the optimized DARTEL approach. Covariates included age and an estimate of total global GMV. Differences were considered significant at p≤0.001, with a whole brain FWE-corrected cluster probability of p<0.025. Among NDs compared to Controls less GMV was observed in the thalamus and bilateral cerebellum and greater GMV was observed in the bilateral putamen and right parahippocampus. Lower thalamic GMV was observed in both female and male NDs compared to Controls. Female NDs also had lower GMV in the left cerebellum and in the ventral medial and orbitofrontal cortices with no areas of greater GMV. Male NDs had lower GMV in bilateral cerebellum and greater GMV in bilateral parahippocampus and left putamen. Within male NDs, GMV in the left putamen was correlated with number of pack years. This study, conducted in a large cohort, contributes to our knowledge of brain morphology in nicotine addiction and provides additional evidence of sex-specific effects on GMV in NDs. Identifying brain vulnerabilities with respect to sex provides a methodological framework for personalized therapies to improve relapse rates for both sexes.

Nipping Cue Reactivity in the Bud: Baclofen Prevents Limbic Activation Elicited by Subliminal Drug Cues

Relapse is a widely recognized and difficult to treat feature of the addictions. Substantial evidence implicates cue-triggered activation of the mesolimbic dopamine system as an important contributing factor. Even drug cues presented outside of conscious awareness (i.e., subliminally) produce robust activation within this circuitry, indicating the sensitivity and vulnerability of the brain to potentially problematic reward signals. Because pharmacological agents that prevent these early cue-induced responses could play an important role in relapse prevention, we examined whether baclofen—a GABAB receptor agonist that reduces mesolimbic dopamine release and conditioned drug responses in laboratory animals—could inhibit mesolimbic activation elicited by subliminal cocaine cues in cocaine-dependent individuals. Twenty cocaine-dependent participants were randomized to receive baclofen (60 mg/d; 20 mg t.i.d.) or placebo. Event-related BOLD fMRI and a backward-masking paradigm were used to examine the effects of baclofen on subliminal cocaine (vs neutral) cues. Sexual and aversive cues were included to examine specificity. We observed that baclofen-treated participants displayed significantly less activation in response to subliminal cocaine (vs neutral) cues, but not sexual or aversive (vs neutral) cues, than placebo-treated participants in a large interconnected bilateral cluster spanning the ventral striatum, ventral pallidum, amygdala, midbrain, and orbitofrontal cortex (voxel threshold p < 0.005; cluster corrected at p < 0.05). These results suggest that baclofen may inhibit the earliest type of drug cue-induced motivational processing—that which occurs outside of awareness—before it evolves into a less manageable state.

Modulation of resting brain cerebral blood flow by the GABA B agonist, baclofen: A longitudinal perfusion fMRI study

BACKGROUND Preclinical studies confirm that the GABA B agonist, baclofen blocks dopamine release in the reward-responsive ventral striatum (VS) and medial prefrontal cortex, and consequently, blocks drug motivated behavior. Its mechanism in humans is unknown. Here, we used continuous arterial spin labeled (CASL) perfusion fMRI to examine baclofens effects on blood flow in the human brain. METHODS Twenty-one subjects (all smokers, 12 females) were randomized to receive either baclofen (80 mg/day; N=10) or placebo (N=11). A five minute quantitative perfusion fMRI resting baseline (RB) scan was acquired at two time points; prior to the dosing regimen (Time 1) and on the last day of 21 days of drug administration (Time 2). SPM2 was employed to compare changes in RB from Time 1 to 2. RESULTS Baclofen diminished cerebral blood flow (CBF) in the VS and mOFC and increased it in the lateral OFC, a region involved in suppressing previously rewarded behavior. CBF in bilateral insula was also blunted by baclofen (T values ranged from -11.29 to 15.3 at p=0.001, 20 contiguous voxels). CBF at Time 2 was unchanged in placebo subjects. There were no differences between groups in side effects or cigarettes smoked per day (at either time point). CONCLUSIONS Baclofens modulatory actions on regions involved in motivated behavior in humans are reflected in the resting state and provide insight into the underlying mechanism behind its potential to block drug-motivated behavior, in preclinical studies, and its putative effectiveness as an anti-craving/anti-relapse agent in humans.